1. Field of the Invention
The present invention is broadly concerned with magnetic switches of the type used as a part of alarm systems for detective relative movement between first and second structural members such as a door and door frame or a window and window frame. More particularly, the invention is concerned with such switches which are especially designed to defeat attempted unauthorized external magnetic manipulation thereof. The magnetic switches of the invention include first and second spaced apart electrically conductive switch elements typically within an enclosed housing and including a permanently magnetized body adjacent the contacts which can be shifted by virtue of magnetic attractions between a first switch-closed position where the body simultaneously contacts both of the switch elements, and a switch-open position where the body is out of contact with both of the switch elements.
2. Description of the Prior Art
Prior art security alarm systems often make use of magnetic switches attached to doors and windows and integrated with the system for detecting unauthorized openings. One common type of magnetic switch used in these situations is a so-called reed switch. It has been found that reed switches are subject to unauthorized manipulation through use of an external magnet. Specifically, an intruder can hold a relatively strong magnet adjacent the reed switch which will then be operated (to either open or close depending on the control scheme). With this accomplished, an intruder can open the door or window without triggering the alarm system.
A number of magnetic switches have been proposed in the past to overcome the inherent deficiencies of reed switches. U.S. Pat. Nos. 5,997,873, 5,530,428, 5,332,992, 5,673,021, and 5,880,659 describe switches of this type.
The present invention is directed to improved magnetic switches for detecting relative movement between first and second members such as doors/door frames or windows/window frames, and normally are used to detect when one of the members is moved from a first position in close adjacency with the second member, to a second position where the one member is moved to a separated open position. Broadly speaking, the magnetic switches of the invention include a switch assembly for mounting to the first member and having first and second switch elements in spaced relationship to each other, an electrically conductive permanently magnetized body shiftable between a first body position where the body is in simultaneous contact with both of the switch elements, and a second body position where the body is out of contact with both of the switch elements. The switch assembly also includes a first magnetically attractive component adjacent the contacts in the first structural member and a second magnetically attractive component for mounting to the second member. Importantly, the first and second attractive components are selected and located so that, when the first and second structural members are in the first, adjacent position, the body will be shifted to a position in simultaneous contact with said first and second switch elements by virtue of a magnetic attraction between the body and the second attractive component; moreover, when the first and second members are in the second, separated position, the body will be shifted to a position out of contact with both of said switch elements by virtue of a magnetic attraction between the body and the first attractive component.
In preferred forms, the switch assembly includes a housing presenting a closed, hermetically sealed chamber defined by a circumscribing sidewall, a concavo-convex bottom wall and a top cover. The top cover includes a relatively weak first attractive component, whereas the bottom wall has a contact surface which defines the second switch element. An elongated, electrically conductive electrode extends downwardly through the top cover and into the chamber, and effectively defines the first switch element. The shiftable body is preferably in the form of a substantially spherical ball formed of permanently magnetized material such as a samarium-cobalt alloy having an external coating of nickel.
The second attractive component for attachment to the second structural member is preferably formed of ferromagnetic material, and may be either a relatively strong permanent magnet or a ferromagnetic plate.